When the editor starts, it opens a file called the scroll file. It also
allocates a 12K data buffer.
SELECT |
 |
When a SELECT is executed, the editor pulls in data from the table
with BULK FETCH statements,
12K bytes at a time. If more than one
BULK FETCH is needed, it appends the data from the previous
FETCH to the scroll file. After the last FETCH in a sequence of
FETCH statements, the data is appended to the scroll file, and the first 12K
of data is read back into the data buffer. If all the data fits in the
buffer, the scroll file is not used at all. Before control returns
to the user, the transaction is ended.
The window pointer is then set to the first row in the buffer, and the
first set of rows is displayed. Cursor positioning and page scrolling,
since they
are handled completely by the editor without accessing the database, are
very fast.
DELETE |
|
When a DELETE is executed, the tuple is fetched from the table based
on its old primary key value. If the tuple is not found, then the
following message is returned:
The row is marked as deleted in the memory buffer, and the
table is redisplayed starting from the window pointer. All rows marked
as deleted are skipped by the display manager.
If the tuple to be deleted is still found in the table, it is compared
against the old value of the tuple. If they are identical, the tuple is
deleted from the table, the row is marked as deleted in the
memory buffer,
and the table is redisplayed starting from the window pointer.
If the tuple in the database has changed, then the following message
is returned:
The tuple's value in the buffer is updated, and the table
is redisplayed starting from the window pointer.
In all three cases, the transaction is ended before control returns to
the user. Locks are obtained and released before additional terminal
I/O is requested.
UPDATE |
|
When an UPDATE is executed, the tuple is fetched from the table
based on its old primary key value. If the tuple is not found, then the
following message is returned:
The row is marked as deleted in the memory buffer, and the
table is redisplayed starting from the window pointer.
If the tuple is found, it is compared against the old value of the
tuple. If they are identical, the tuple is updated, the tuple's value
in the buffer is updated, and the table is redisplayed starting from the
window buffer.
If the tuple in the database has changed, then the following message
is returned:
The tuple's value in the buffer is updated, and the table
is redisplayed starting from the window pointer.
In all three cases, the transaction is ended before control returns to
the user.
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 | CAUTION:
A dirty bit is kept for the whole data buffer. The bit is set when the
contents of the buffer is modified (that is, becomes "dirty").
If set, the current contents have
to be written to the scroll file before a new 12K block is read from
it. |
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